This invention relates to pumps for liquids, and more particularly to a self-priming pump for swimming pools and the like.
The centrifugal pump is well known as an efficient means for moving large quantities of liquid against a head of moderate pressure. The centrifugal pump comprises a shaft-driven impeller to which liquid is introduced proximate to its rotational axis. Vanes on the rotating impeller urge the liquid outwardly, discharging the liquid at high velocity into a diffuser associated with a casing for the impeller. Liquid pressure is produced by the pump from the centrifugal force produced on the liquid, and by conversion of the dynamic pressure associated with the high velocity into static pressure as the velocity is gradually reduced in the diffuser. A significant limitation of centrifugal pumps, and other pumps that do not provide positive volumetric displacement, is that, when empty, they do not remove air from the suction pipe, but once the air is removed by other means, it is "primed" and pumps normally.
The most common self-priming pump, known as the "wet self-priming pump" has means for traps liquid in the body of the pump. Once the pump has been primed by conventional means, sufficient fluid remains in the body that liquid can be drawn into the pump when it is started from a level a few feet below that of the pump.
The wet self-priming pumps of the prior art suffer from several disadvantages, including:
1. The preliminary priming is awkward and time consuming;
2. Provisions for the preliminary priming are bulky, expensive, and subject to harmful and/or incapacitating air leaks;
3. The self-priming is slow and unreliable, such that the pump may run dry indefinitely;
4. Elaborate, expensive impeller seals are required for significant self-priming capability; and
5. Expensive shaft seals are required to withstand high-temperatures produced during the long periods of dry operation during priming.
Positive displacement pumps such as rotary vane and gear pumps are self-priming to the extent that atmospheric pressure in excess of the liquid vapor pressure is sufficient to raise the liquid level in the suction pipe to the height of the pump. However, they are not as efficient as centrifugal pumps; therefore, they are not generally used for pumping large volumes of a liquid against moderate head of pressure, even when priming is a consideration.
Another disadvantage of many liquid pumps of the prior art is that they are difficult to start after priming, due to the inertia of the liquid column that must be accelerated. Thus expensive, oversized motors and/or special starting provisions are required, with excessive electrical power drain.
Thus there is a need for an efficient, effective self-priming liquid pump that does not require preliminary priming and is easy to start.